Barrier layers and silicide layers are often an integral part of semiconducting devices. Materials which function as barriers to metal diffusion may be incorporated in metal interconnect structures that are part of integrated circuits (ICs). Barriers to metal diffusion are typically required to generate reliable devices, since low-k interlayer dielectrics typically do not prohibit metal diffusion.
Silicide contacts are of specific importance to IC's, including complementary metal oxide semiconductor (CMOS) devices because of the need to reduce the electrical resistance of the many Si contacts, at the source/drain and gate regions, in order to increase chip performance. Silicides are metal-silicon compounds that are thermally stable and provide for low electrical resistivity at the Si/metal interface. Silicides improve resistivity by providing ohmic contacts to the Si surface during silicide formation. Reducing contact resistance improves device speed therefore increasing device performance.
Forming devices having a titanium oxynitride diffusion barrier atop a cobalt silicide has presented difficulty when processed with corresponding tungsten vias. First, a cobalt silicide region is formed by depositing cobalt atop a silicon substrate and then annealing the structure to promote interdiffusion of cobalt with the substrate. The unreacted cobalt is then stripped and a layer of titanium is deposited atop the cobalt silicide. The titanium layer is conventionally processed to produce a silicide contact and a diffusion barrier.
More specifically, titanium is deposited atop the silicide and then treated by a nitrogen-containing forming gas anneal. The nitrogen-containing forming gas anneal produces a titanium nitride layer, or a titanium oxynitride layer, at a top surface of the titanium metal layer and produces an amorphous titanium cobalt oxygen silicide layer at the interface between the titanium layer and the cobalt silicide.
Referring to FIG. 1, the amorphous titanium cobalt oxygen silicide 5 is formed in regions in which titanium diffusing down from the titanium layer reacts with the underlying cobalt silicide 6 and layer of SiOx on Co silicide surface. A remaining unreacted portion 8 of the titanium layer is positioned between the amorphous titanium cobalt silicide 5 and the titanium nitride layer 7. Due to differing atomic radii of titanium and the silicon, of the cobalt silicide 6, they cannot replace each other in the atomic structure and therefore produce an amorphous layer. During conventional annealing, a thick amorphous titanium cobalt oxygen silicide layer 5 is formed. The increasing thickness Tα of the amorphous titanium cobalt oxygen silicide 5 results in a thinned (Tβ) titanium nitride layer 7. Thinning of the titanium nitride layer 7 occurs because as the titanium diffuses and is adsorbed in the cobalt silicide 6 the amount of titanium available for forming the titanium nitride diffusion barrier 7 is reduced. If the amorphous layer adsorbs too much titanium and becomes greater than 4.0 nm in thickness Tα, the titanium nitride layer 7 becomes too thin to be effective as a barrier. The titanium nitride diffusion barrier 7 must maintain a thickness to be effect as a barrier to protect the underlying silicide layer 6 during later processing steps.
Following the formation of the titanium nitride diffusion barrier, tungsten interconnects are formed by depositing tungsten through chemical vapor deposition with a tungsten hexaflouride precursor gas. If the titanium nitride diffusion barrier layer is too thin to be an effective barrier to fluorine, the fluorine may attack the underlying amorphous titanium cobalt silicide layer resulting in a ruptured via producing an electrically open circuit.
The above phenomenon is equally applicable to nickel silicide and barrier layers comprising hafnium or zirconium, where the difference in atomic radii of the elements in the barrier layer and underlying silicide results in the formation of an amorphous silicide interlayer.
In view of the above, a method for forming a via interconnect to a silicide region having an effective diffusion barrier positioned between the via interconnect and the underlying silicide is needed.